Hot melt adhesives are applied to a substrate when molten, then placed in contact with a second substrate. The adhesive cools and hardens to form a bond between the substrates. Hot melts are widely used for industrial adhesive applications such as product assembly and packaging. The latter include case and carton sealing.
Hot melt adhesives are widely used in packaging industry to seal cardboard cases, trays, and cartons. Some of these operations require hot melt adhesive with exceptional high heat resistance (ability to maintain fiber tear at high temperature) without compromising good cold temperature performance. One example is to package freshly baked goods at temperature greater than 140° F. and its subsequent storage at freezer temperatures of −20° F. Another example that requires hot melt adhesives with good heat and cold resistance is in the process of shipping sealed cases, cartons or trays by trucks or railway. The temperature in a truck could be very high (up to 145° F.) in the summer or very low in the winter (−20° F.). The hot melt adhesives used should be strong enough such that the sealed containers will not pop open during the transportation process.
Hot melts for packaging applications such as case and carton sealing are typically composed of a polymer, a diluent (usually a tackifier) and a wax. The wax serves several functions. Because it is low in molecular weight, it reduces viscosity. Low viscosity helps to lower application temperatures, provide cleaner processing, and also good wet out of the substrates. In addition, the wax crystallizes quickly which helps the material to harden or set quickly. Fast set speed is important for high speed production. Lastly, the wax provides temperature resistance to the final bond due to its elevated melting point.
Conventional packaging adhesives use petroleum-derived waxes such as paraffin and microcrystalline wax. The lower molecular weight of paraffin wax, makes it the primary choice for low application temperature adhesives.
While hot melt adhesives based on ethylene vinyl acetate and/or ethylene n-butyl acrylate having been used in packaging applications, there continues to be a need for new and improved hot melts formulations, including formulations based on new polymers, or particular combinations of polymers that possess both high heat resistance and good cold adhesion. The current invention addresses this need.